This invention provides a composition and method of using that composition in the recycle of aluminum, especially scrap aluminum such as found in used beverage containers. More particularly, this invention relates to a high purity salt flux composition and additive composition for use with a high purity salt for making the high purity salt flux composition, which salt flux composition can be used during the remelting of scrap aluminum. The method of the invention is directed to using the high purity salt flux composition during the aluminum recovery process to improve coalescence of the molten aluminum.
Use of molten salt fluxes in the secondary aluminum industry is known to improve direct recovery of aluminum in remelting processes. Aluminum and scrap aluminum, such as used beverage containers (UBCs), are treated using such processes. Remelting of the aluminum in a furnace is carried out under cover of a layer of molten salt to prevent oxidation of the aluminum in the furnace atmosphere and to promote coalescence of the molten aluminum so as to maximize recovery of aluminum. During processing, an oxide film tends to form on the surface of the molten aluminum droplets. The oxide film inhibits coalescence of the molten aluminum, causing smaller particles to be lost in the process thereby reducing the amount of aluminum recovered. The unrecoverable aluminum droplets having the oxide film are sometimes referred to as dross.
Use of a salt flux in the furnace helps to strip away and suspend the oxide film so that coalescence of the droplets increases and dross formation decreases. The salt flux wets the oxide film and initiates disintegration of the film, stripping it from the surface of the molten aluminum droplets. Fragments of the oxide film stripped from the aluminum remain suspended in the flux. The aluminum droplets, which have a density greater than the flux, then form a continuous molten pad beneath the flux layer. The flux also prevents further oxide formation by keeping the metal protected from the atmosphere of the furnace.
One type of salt flux is primarily composed of a mixture of high purity sodium chloride and potassium chloride. The high purity salts used in such processes are solution mined and purified by complex, highly developed methods.
An object of this invention is to provide for improved aluminum recovery in a recycle process which includes scrap aluminum and salt flux.
Another object of the invention is to provide for improved coalescence of the molten aluminum in a process which includes melting scrap aluminum with a high purity salt flux composition, especially aluminum from UBCs.
Yet another object of the invention is to use an additive with a high purity salt flux composition, which additive includes an alkaline agent and a fluoride source to increase the coalescence of the remelted molten aluminum, thereby improving recovery of the metal.
Further objects and advantages of the invention will be found by reference to the following specification.
The present invention is directed to a method and composition for increasing aluminum recovery in the recycle of aluminum, and in an important aspect, scrap aluminum such as aluminum from used beverage containers (UBCs). The salt flux composition of the invention protects the molten aluminum from oxidation, strips a protective oxide film from the molten aluminum so that molten aluminum droplets can coalescence and holds the oxide film in suspension so that the molten aluminum can be recovered.
Broadly, the salt flux composition of the invention comprises high purity NaCl and/or KCl, an alkaline agent and a fluoride source. The amounts of alkaline agent and fluoride source, along with the high purity NaCl and/or KCl, are effective for improving coalescence and reducing aluminum loss in the recovery of aluminum from molten scrap aluminum, where the improvement is relative to a salt flux composition which comprises a high purity salt without an alkaline agent and a fluoride source.
In an important aspect, the invention includes a salt flux composition which comprises high purity NaCl and/or KCl, an alkaline agent and a fluoride source, where the amounts of the alkaline agent and fluoride source, along with a high purity salt in the salt flux composition, are effective for improving coalescence and reducing aluminum loss in the recovery of aluminum from molten scrap aluminum, especially UBCs which comprise specific alloys of aluminum. Such an improvement is relative to a process using like conditions and a high purity salt flux composition consisting essentially of a high purity salt without the alkaline agent and fluoride source. Generally, the salt flux composition comprises at least about 0.2 weight percent and preferably from about 0.2 to about 3 weight percent alkaline agent and at least about 3 preferably from about 3 to about 7 weight percent fluoride source, all based upon the weight of the salt flux composition. Generally, the salt flux composition comprises from about 90 to about 96.8 weight percent NaCl and/or KCl in this aspect of the invention.
The high purity salt flux will have less than about 0.2 weight percent sulfate. The composition and process of the invention also will at least maintain or improve aluminum coalescence as compared to a process using a salt flux consisting essentially of a high purity alkali metal salt such as NaCl and/or KCl.
In another aspect, the invention is directed to an additive composition which consists essentially of an alkaline agent and fluoride source for addition to high purity NaCl and/or KCl to provide a high purity salt flux composition which improves aluminum coalescence in a process which involves mixing scrap aluminum metal, the additive and high purity salt to recover aluminum, as opposed to a process in which aluminum is recovered from dross resulting from previous aluminum processes without the further addition of scrap aluminum.
In an important aspect, the alkaline agent is selected from the group consisting of Na2CO3, NaOH, KOH, K2CO3 and mixtures thereof and the fluoride source is selected from the group consisting of MF, CaF2, MAlF4 M3AlF6 and mixtures thereof, where M is K or Na.
In another important aspect, the additive composition comprises alkaline agent in an amount of from about 3 to about 50 weight percent and fluoride source in an amount of from about 50 to about 97 weight percent, each based upon the weight of the additive composition. In another important aspect, the additive composition comprises between about 4 to about 10 weight percent of the salt flux composition.
The salt flux composition of the invention generally is used in the process of the invention at a level of at least about 1 weight percent, based upon the weight of aluminum being processed.